My fairly young MacBook Pro started randomly hanging, not coming out of sleep, and being generally disagreeable a couple days ago, and it turned out to be a bad hard disk.

What’s disconcerting is that Disk Utility.app didn’t see any problem with the disk. I had to install smartmontools to find the error.

After installing MacPorts, install smartmontools:

sudo port install smartmontools

Then tell the drive to do a long self-check in the background:

sudo smartctl -t long /dev/disk0

Note that this check may take an hour to run, but it’s done in the background, so you can continue to use your computer while it does its little dance on the catwalk.

Check the status of the test with:

sudo smartctl -c /dev/disk0

I was unlucky:

...
Self-test execution status: ( 121) The previous self-test completed having
  the read element of the test failed.
...

See The macosxhints forums for more discussion about this issue.